This invention relates to a method and apparatus for fault detection and isolation and more specifically to the interruption of power to an electronic circuit when a short circuit occurs between a circuit power line and ground.
As is known in the art, circuit boards are typically mounted on a back plane and are powered with a common power supply. Power is distributed onto the back plane with power supply lines. Plugs are mounted on the back plane and connect to these power supply lines. Engaging with the plugs on the back plane are circuit boards with edge connectors. Power is fed from the power supply lines through the plugs on the back planes and into the respective circuit boards.
Under certain conditions, short circuits may develop on the circuit boards, causing circuitry on the board to fail. For example, moisture may develop across two adjacent circuit etches or integrated circuit leads. Electric current may then flow between the etches or leads, resulting in a short circuit. A short circuit may also occur by a technician inserting a circuit board into the back plane at a non-conventional angle. If this happens, the fingers on the plugs may short across the etches on the board. Further then testing a circuit board, the technician may touch two adjacent leads or etches on the board with an oscilliscope probe, resulting in a short circuit. A short circuit may also occur when a faulty component, such as defective circuit board, integrated circuit, resistor, or capacitor, has an internal short circuit.
If a circuit board cannot be isolated from a main power line when it occurs, the voltage level on the main supply line may drop below a predetermined level, thereby interrupting power to the other circuit boards. This failure may result in the system being brought down or the system's components operating outside their specified operating parameters. Operating components outside their specified operating parameters may reduce their life expectancy and may cause them damage. In order to prevent system failures and component damage, circuit boards are typically protected by a circuit breaker, or a fuse disposed on each circuit board. When a failure occurs, the fuse or circuit breaker blows or trips, thereby disconnecting power to the faulty circuitry.
Fuses and circuit breakers have many drawbacks. First, when fuses blow, they must be replaced. Accordingly, replacement fuses must be available to fix the circuit board. Second, fuses have the drawback of requiring manual replacement by a technican, as the circuit board must often be removed to replace the part. Circuit breakers have the drawback that once a circuit breaker trips, it remains in the trip position until it is manually reset. Accordingly, when the failure that caused the circuit breaker to trip is corrected, the circuit breaker remains in the trip position. Thus, a technician may have to be on-site to manually reset the circuit breaker. Another drawback is that when fuses are replaced, the technician may inadvertently replace the fuse with one of a higher amperage to prevent repeated fuse blowing. This higher amperage replacement may endanger the faulty board even further, as components on the board may be damaged before the fuse blows.